The Earth is an interacting system of matter and energy, that as part of its
functioning produces phenomena like volcanoes, glaciers, mountain ranges,
oceans, and continents. The energy that keeps this system going is on one
hand the internal heat (from radioactive decay) that drives plate tectonics, and
on the other hand solar energy that maintains ocean and atmosphere circulation
and helps to drive erosion.

In the previous chapter we have discussed the internal processes (radiogenic
heat production, mantle convection) that drive plate tectonics and crustal
dynamics. Geologists call this complex of internal processes the Tectonic System.
They refer to the complex of solar powered external processes (atmosphere and
ocean circulation, erosion) as the Hydrologic System, because
water and its properties are the key ingredient.

THE HYDROLOGIC SYSTEM includes the entire cycle (hydrologic cycle) of
water movement on the Earth (be it on the surface or in the air). The driving force behind
this cycle is the energy input from the sun.

THE HYDROLOGIC CYCLE as shown in above diagram shows the various
pathways of (1) water to the oceans (rivers, glaciers, precipitation); (2) water into the
atmosphere by evaporation (from falling rain, rivers & lakes, soil, the oceans,
transpiration by plants); and (3) onto the landmasses (by rain, snow). Water
movement/transport occurs through movement of clouds, by rivers, ocean circulation,
groundwater flow, and evaporation.

The bulk of the water is contained in the oceans, which contain about 30000 times more
water than atmosphere and continents combined, cover approximately 70% of the Earth's surface
and are on average 3800 m deep. The remainder of the water is found in ice caps
& glaciers (3%), groundwater (1%), and rivers and lakes (0.01%). The
latter two reservoirs constitute the terrestrial fresh water supply. Thus,
only a very
small fraction of the overall water supply is suitable and available for human use.
The water transfer between these reservoirs is accomplished by the processes of
evaporation, transpiration, precipitation, and flow of water (following
gravity).
Two minor reservoirs by size, the atmosphere (0.001%) and the biosphere (the
totality of living tissue; 0.0001%) play a role that is not reflected by their
size. As we shall see below, cycling of water through the atmosphere is an
important factor for energy transfer in the atmosphere, and in Chapters 7
through 11 we will look at the important role that organisms play in driving the
evolution of the climate and atmosphere evolution through the production and
consumption of the greenhouse gases carbon dioxide and methane.

Every year about 30000 to 40000 cubic kilometers (a cube 30-35 km in size) of water
move across the surface of the continents to the oceans, profoundly shaping the surface of
the continents. Evaporation by the sun effects lifting of water into the atmosphere, and
the counterforce to this process is gravity that forces rain to fall back on the earth
and causes water move back to the oceans in streams (river systems), on the way
eroding soils, cutting canyons, and transporting solids (silt, sand, clay) and dissolved
salts to the oceans. The transfer of water vapor from the oceans to the atmosphere goes
hand in hand with the transfer of tremendous amounts of thermal energy to the atmosphere
and is very important for atmospheric circulation (see below). For this reason atmospheric
circulation and winds can be considered part of the hydrologic cycle.

In cold climates water in the form of glaciers (glacial systems) moves
downslope
due to gravity, erodes bedrock by abrasive action, and transports sediment. Glaciers may
either enter the sea directly, or melt at the end and add their water to the continental
surface runoff.

Water may also seep into the porous portions of the earth's surface and is then called
groundwater (groundwater systems). It may dissolve rocks in the subsurface and
create karst systems
(caves in limestone) and sinkholes.

Along the shores of oceans (shoreline systems)
water that is agitated by waves
(wind, atmospheric circulation) erodes and moves sediment and forms beaches, sandbars,
lagoons, and related coastal features.

Winds (eolian system) can carry considerable amounts of sand and dust and can
profoundly shape landscapes (sand seas of deserts) as well as human destiny (the highly
fertile loess plains of southern China, without them it would be much more difficult to
feed a population of 1 billion people).

Thus,the hydrologic cycle affects a broad range of landscape-shaping dynamic
systems. We therefore consider river systems, glacial systems, groundwater systems,
shoreline systems, and eolian systems as subsystems of the hydrologic system.In
terms of overall importance for the shaping of the Earth surface, river systems are the
primary force. Glaciers, although very powerful in modifying the land surface, were
only abundant for brief intervals in Earth history.

Chemical Energy (holds together atoms in molecules, the energy of the
chemical bond)

Nuclear Energy (released during radioactive decay)

All those various forms of energy occur and are transformed into each other as geologic
processes proceed (kinetic energy to heat during meteorite impact; thermal energy to
chemical energy and vice versa during melting and recrystallization; thermal energy into
gravitational potential energy [sun evaporates water and forms clouds] - gravitational
potential energy into kinetic energy [rain drops fall]).

The total amount of energy that moves through the system is huge. It is on the order
of 174 ,000 terawatts (1 terawatt = 1012 watts [energy per time]; 1 horsepower
= 746 watts). Almost all of this energy comes to us via solar radiation.
The
Earth system receives 5000 times more energy from the sun than from the interior of the
planet. Thus, although its manifestations are impressive (mountain ranges,
earthquakes, volcanoes), the internal energy that keeps up mantle convection and drives
the tectonic plates is only a small fraction of the energy that moves through the system.
To put it into perspective, the total energy production by humans is about 9.6
terawatts at any given time. Thus, there is plenty of energy in the system.
Even if humans were to extract all their energy needs from the solar input, there would
still be plenty of energy left to keep the planet going.

Solar energy input dominates the surface processes (wind, weather,
climate, ocean circulation, etc.) of the Earth, and because the Earth is a sphere, its
input is not uniform across the planet. The concentration of solar energy per unit
area depends on the angle at which the solar radiation arrives. In equatorial
regions, where the sun's rays come in more or less perpendicular, a maximum amount of heat
is received. In polar regions, on the other hand, the sun's rays come in slanted at
a shallow angle and considerably less heat is received (see diagram below).

Average Earth surface temperatures. Blue indicates lowest temperatures (polar
regions), red indicates highest temperatures (around the equator). The data used for
this diagram were collected between January 1985 and December 1992.

As a consequence, the polar regions stay cooler. Just like in our hot water pot that was heated from
below, the uneven global heat distribution gives rise to convection currents that attempt
to equalize the heat distribution.

In its most simple form we can imagine that heated air at the equator rises up, and
spreads north and south towards the poles. It gradually cools, sinks down in the
polar regions, and then flows across the Earth surface to the equator. There it
heats up again and the convective cycle is repeated. This is what would happen if
the Earth did not rotate. Rotation adds another twist to the story, because the
Coriolis
Effect makes the northward flowing currents veer off course (to the right,
in the direction of rotation).
The Coriolis Effect breaks the air exchange between equator and poles into three
circulation belts. We have seen this effect also in the atmospheres of
large gas planets in Chapter 2.

An idealized view of global air circulation.
Along the equator, surface winds converge,
bringing with them abundant moisture.
Thus, clouds are abundant in this area
and it
rains a lot. Between 20 and 35 degrees
latitude, descending dry and warm air
masses give rise to cloudless skies and
desert climates. Along the polar front, rising
air masses again give rise to cloud formation
and rain.

An important aspect of global air circulation is the movement of water through
the atmosphere. Water has a much higher heat capacity than the atmospheric
gases. Its heat capacity is approximately four times that of air (heat capacity of
water = 1 cal/gm/deg). In addition, because water changes from the liquid to the gas
state (evaporation of water) and back (condensation into clouds) as it gets cycled through
the atmosphere, we also have to consider the latent heat of vaporization (540 cal/gm).
This is the amount of heat necessary to get the water molecules into the gas
state. For example, if we evaporate a liter (1 kg) of water, 540,000 calories of
latent heat are stored in the vapor. Once the water condenses, this latent heat is
released, and heats up the surrounding air. If we assume for example that this water
vapor is contained in 1000 kg of air (roughly 1000 cubic meters, or a cube of 10 meters
size), it would heat up this air mass by about two degrees as it condenses. Air at
30 degrees C (86 degrees F) can hold up to 30 grams of water vapor per cubic meter (or
kilogram) of air. If this water vapor condenses it releases 16,200 calories of
latent heat. This in turn can cause the air to heat up as much as 64 degrees C.
It is just like heating the air up in a hot air balloon. The air will expand
and rise (think of rising and billowing clouds before a thunderstorm). For these
reasons, condensing or vaporizing water has a profound effect on atmospheric circulation,
and moist air masses are prone to very powerful climatic disturbances (e.g. thunderstorms, tornadoes, hurricanes).

Water vapor is constantly cycling through the atmosphere, evaporating from the surface,
condensing to form clouds blown by the winds, and subsequently returning to the Earth as
precipitation. Heat from the Sun is used to evaporate water, and this heat is put into the
air when the water condenses into clouds and precipitates. This evaporation-condensation
cycle is an important mechanism for transferring heat energy from the Earth's surface to
its atmosphere and in moving heat around the Earth.

This image shows global humidity patterns (April 27, 1995, at 1745 UTC, measured by the
GOES 8 satellite). The areas with the lowest humidity (brownish) are those that have
the weakest circulation and were feared by sailors because of a lack of sustained winds.

Water also has a substantially higher heat capacity than rocks (by a factor of five),
and therefore the oceans can store much more heat than the land surfaces of the planet.
Because the oceans also cover about 70% of the Earth surface and are on average 3.8
km deep, they are the major heatsink of the planet and serve as temperature buffers for
the ocean/atmosphere system. The bulk of the thermal energy at the Earth surface
is stored in the oceans. The large thermal inertia of the oceans is a key factor in
stabilizing Earth's climate.

Because the oceans are of course also heated more intensely in the equatorial regions,
there is abundant warm water near the equator, and cold water in the polar regions.
The atmospheric circulation that is set up between the equator and the poles also
influences the redistribution of these water masses. Wind blowing over the ocean
surface exerts drag (friction) and starts to move the surface waters. In addition
the currents are influenced by the Coriolis Force and the tides. A general pattern of
ocean current is shown in the map below.

The currents are also influenced by the position of landmasses. Whereas in the
Pacific we have currents that more or less correspond to the patterns of the surface
winds, current patterns look more complex in the Indian Ocean and the Atlantic. In
the North Atlantic, the Gulfstream moves warm equatorial surface waters northward, and a
return current of cold arctic water moves southward across the Atlantic seafloor.

Decades of research on ocean currents have revealed that there is a large scale oceanic
circulation system. The Gulfstream that moves into the North Atlantic finally sinks
as it cools and returns south across the Atlantic seafloor, flows as a bottom current into
the South Atlantic, and then rises to the surface again in the Indian Ocean and the
Eastern Pacific, only to warm up (absorbing heat from overlying air masses) and turn west
to feed back into the Gulfstream. Actually, the sinking of the Gulf Stream is not so
much a consequence of cooling and density increase, but one of salinity. Equatorial
surface waters are more saline (high evaporation), and the waters of the North Atlantic
are less saline (because they mix with meltwater [low salinity] from icebergs and from the
Greenland ice cap). Thus, once the Gulfstream cools sufficiently it is heavier that the
lower salinity North Atlantic seawater and sinks.
A complete run through this
current system is estimated to take about 1000 years.

At the moment this current system appears very stable, but comparatively minor changes
may upset the delicate balance of temperature differences and prevailing winds. For
example, scientists have speculated that
melting of the Greenland icecap could introduce
so much meltwater (of low salinity and low density) to the North Atlantic that the
Gulfstream is prevented from sinking and returning as a southward flowing bottom current.
This would interrupt the current system and cause equatorial waters (and regions)
to get warmer, and northern Europe to get colder. Spock
says....

Atmospheric and oceanic circulation are closely coupled. For example, changes in
the pattern of sea surface temperatures are reflected in changes of wind patterns around
the globe (water masses transfer or absorb heat to/from overlying air).
Specific examples for this kind of interaction are weather phenomena knows as El Nino and
La Nina (these will be discussed later).

On the modern Earth, the Tectonic System, the Hydrologic System (atmosphere &
hydrosphere), and organic life (biosphere) interact in complex ways, and matter and energy
is exchanged between these subsystems. The net result of all these interactions,
when averaged over periods of decades to centuries, is the Earth System's overall
"condition", a state of dynamic equilibrium. The global climate is a major
expression of this "condition".

Although circulation of air masses and ocean waters (as outlined above) plays a
significant role in determining global heat exchange and average world temperatures, there
are several other, more fundamental, controls on climate. The basic items that
determine Earth climate are:

Astronomical Parameters

Distribution of Continents and Oceans

The Greenhouse Effect

Item 2 (Distribution of
Continents and Oceans) is most important because it determines the routing of global current systems and therefore the
intensity/strength of the oceanic heat exchange between equatorial and polar regions.
The movement of the continents across Earth's surface modulates the global climate
on very long time scales by changing wind patterns, ocean currents, and the ease with
which ice can accumulate. Under some configurations of continents and oceans the heat
exchange may be severely impeded, leading to hot tropics and ice covered polar
regions. Also, when we have all the landmasses bunched up in the polar regions the
warm ocean current may not get far enough north and we end up with glaciated
continents. Conversely, when we have all the continents hanging around the equator,
the polar regions have very good oceanic heat exchange and world temperatures would be
more uniform.

Item 3 (Greenhouse Effect) will be discussed at some length later in this lecture.
Basically, this is where linkage to the biosphere comes in.

Item 1 (Astronomical Parameters), will be surveyed now, because we have already set
the stage by going over the origin of the solar system.

Earth's rapid rotation about its axis (compared to the time for one orbit around
the Sun) determines day and night, spreads the incoming solar radiation more or less
uniformly around circles of latitude, and strongly affects circulation patterns in the
oceans and atmosphere.The eccentricity of Earth's orbit modulates the intensity of the incoming solar
radiation through the course of the year. The tilt of Earth's axis with
respect to the ecliptic plane results in the seasons. Note, that it is
summer in the northern hemisphere when the Earth is closest to the sun (perihelion,
January 3rd, 147 million km), and winter when it is farthest away (aphelion, July 4th, 152
million km). Thus, the tilting of the northern hemisphere towards the sun in summer
(longer days, more solar energy input) and the tilting away from the sun in the winter
(shorter days, less solar energy input) are more important than the actual distance to the
sun.

The relationship between the eccentricity of Earth's orbit and the orientation (tilt) of
the planet's axis of rotation vary systematically over time. This causes cyclic
shifts of solar energy input that can produce climate modulations (repetitive temperature
variations). These variations are termed Milankovitch cycles
in honor of Serbian astrophysicist and mathematician Milutin
Milankovitch who in 1920 figured out these variations and proposed that they influence
world climate.
This image shows a tracing of ocean surface temperatures for the past 700,000
years, based on oxygen isotopes of marine microfossils. We see a very clear
periodicity of ca. 100,000 years
(the eccentricity cycle of
Milankovitch), and a superimposed periodicity of ca.
20,000 years (the
precession cycle of Milankovitch).

Fluctuations in the Sun's output
(apparently with a period of 500 to 1000 years) may
produce small changes in climate over intervals of 100 to 200 years. A
time interval of cooler temperatures that ranged from the mid-14th to the
mid-19th century, also known as the "Little Ice Age", may have been caused
this way, but this is a tentative interpretation. The Little Ice Age
brought an end to an unusually warm era known as the Medieval climate optimum,
and brought bitterly cold winters to many parts of the world. It is most
thoroughly documented in Europe and North America. In the mid-17th century,
glaciers in the Swiss Alps advanced, gradually engulfing farms and crushing
entire villages. The River Thames and the canals and rivers of the Netherlands
often froze over during the winter, and people skated and even held fairs on the
ice. In the winter of 1780, New York Harbor froze, allowing people to walk from
Manhattan to Staten Island. Sea ice surrounding Iceland extended for miles in
every direction, closing that island nation's harbors to shipping. The severe
winters affected human life in ways large and small.
Famines became more frequent (the
one in 1315 killing 1.5 million people alone), and
deaths from disease increased.
The population of Iceland fell by half,
and the Viking colonies in Greenland
died out. The Little Ice Age can be seen in the art of the time; for
example, snow dominates many village-scapes by the Flemish painter Pieter
Brueghel the Younger, who lived from 1564 to 1638.

Eventually, all things merge into one, and a river runs
through it.
The river was cut by the world's great flood and runs over rocks from the
basement of time.
On some of the rocks are timeless raindrops.
Under the rocks are the words, and some of the words are theirs.
I am haunted by waters.